Optical mice are commonly used as navigation input devices for computers. Typically, the useful light intensity that is used in optical mice for tracking on a glass surface is very low. Thus, even a small amount of noise or crosstalk can negatively impact the ability to distinguish surface features. One source of crosstalk is light scattered from imperfections in output facets, or surfaces, of the illumination optics (e.g., lenses) between the light source and the glass surface. For example, the finite surface roughness of the output facets of a lens module causes light to scatter in almost all directions. Also, the accumulation of dust and dirt on those surfaces contributes to the light scattering and crosstalk.
FIG. 1A depicts a schematic diagram of a conventional optical navigation device 10. The conventional optical navigation device 10 includes a light source 12 and illumination optics 14. The illumination optics 14 have at least one output surface 16 where the light exits the illumination optics 14. This is the surface at which light can scatter due to optical imperfections and/or accumulation of dust. The conventional optical navigation device 10 also includes an image sensor 18, imaging optics 20 (i.e., a lens), and an imaging aperture structure 22 to define an imaging aperture 24. In operation, these components are arranged relative to a tracking material 26 (e.g., glass) with a thickness, L. The tracking material 26 has at least a primary interface 27 (e.g., the top surface) which is used as a tracking surface. The illustrated tracking material 26 also includes a secondary surface 28 that may impact the performance of the optical navigation device 10. Many types of glass tracking materials have both primary (i.e., front) and secondary (i.e., back) surfaces 27 and 28 that reflect light from the light source 12 toward the image sensor 18.
In general, the illumination optics 14 direct light from the light source 12 toward the tracking surface 27. The combination of the image sensor 18, the imaging optics 20, and the imaging aperture 24 forms a unique field of view, β, through which the image sensor 18 generates navigation images of the tracking surface 27. For convenience, this field of view (FOV) is generally designated as the field of view of the image sensor 18. Also for reference purposes, a reference origin 30 is shown at approximately the center of the field of view of the image sensor 18 at the tracking surface 27. Also for reference purposes, although the field of view is a solid angle in the three-dimensional space, for simplicity only a two-dimensional cross section of that angle is shown in FIG. 1A.
Unfortunately, because of the reflective properties of the tracking surface 27, the navigation images generated by the image sensor 18 can also include noise that interferes with accurate tracking operations. Specifically, the image sensor 18 can detect one or more images 32 and 34 of the light scattering that occurs at the output surface 16 of the illumination optics 14. The upper image 32 is referred to as a primary image because it is formed by reflections from the primary surface 27, while the lower image 34 is referred to as a secondary image because it is formed by reflections from the secondary surface 28.
FIGS. 1B and 1C depict top views of the fields of view of the conventional optical navigation device 10 of FIG. 1A at approximately the primary image 32 corresponding to the primary surface 27 of the tracking material 26 and the secondary image 34 corresponding to the secondary surface 28 of the tracking material 26, respectively. These illustrations show the approximate locations of the light source 12, the image sensor 18, the imaging optics 20, and the reference origin 30. The primary and secondary images 32 and 34 are shown with emphasis on an area 36 that has an intensity above a threshold intensity. A portion 38 (shown hashed) of the area 36 of the primary image 32 overlaps with the field of view 38 of the image sensor 18, and all of the area 36 of the secondary image 34 overlaps with the field of view 38 of the image sensor 18. The detection of one or both of these images 32 and 34 by the image sensor 18 can have detrimental effects on the ability of the optical navigation device 10 to perform navigation operations because these images 32 and 34 make it more difficult to distinguish features of the tracking surface 27 in the resulting navigation images generated by the image sensor 18.